Respiratory gating may reduce the effects of respiratory motion in image reconstruction, such as Emission Computed Tomography (ECT) image reconstruction. A respiratory motion signal may be needed for the respiratory gating. In some embodiments, a data-driven technique may be used to extract a respiratory motion signal. Limited by factors including, for example, the algorithm itself applied in a data-driven technique, the field of view of the image scanner used to acquire image data to be analyzed, the data-driven technique may extract respiratory motion signals that have flipped phases. The direct use of such respiratory motion signals may be troublesome for the determination of inspiration/expiration phases of the respiratory motion. Furthermore, the uncertainty on the inspiration/expiration phases of the respiratory motion may cause inaccurate motion correction of an image.